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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20
Influence of parameters on the photocatalytic degradation of phenolic contaminants in wastewater using TiO2/UV system a
a
a
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Rijuta G. Saratale , Hyun S. Noh , Ji Y. Song & Dong S. Kim a
Department of Environmental Science and Engineering, Ewha Womans University, Seoul, Republic of Korea Published online: 19 Aug 2014.
To cite this article: Rijuta G. Saratale, Hyun S. Noh, Ji Y. Song & Dong S. Kim (2014) Influence of parameters on the photocatalytic degradation of phenolic contaminants in wastewater using TiO2/UV system, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:13, 1542-1552, DOI: 10.1080/10934529.2014.938532 To link to this article: http://dx.doi.org/10.1080/10934529.2014.938532
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Journal of Environmental Science and Health, Part A (2014) 49, 1542–1552 Copyright © Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2014.938532
Influence of parameters on the photocatalytic degradation of phenolic contaminants in wastewater using TiO2/UV system RIJUTA G. SARATALE, HYUN S. NOH, JI Y. SONG and DONG S. KIM
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Department of Environmental Science and Engineering, Ewha Womans University, Seoul, Republic of Korea
The photocatalytic degradation of phenol in aqueous suspension using commercial TiO2 powder (Degussa P-25) irradiated with UV light was investigated. Photodegradation was compared using a photocatalyst (TiO2 alone), direct photolysis (UV alone) and TiO2/ UV in a single batch reactor with mercury lamp irradiation. The study focused on the influence of various operating parameters on phenol treatment efficiency, including catalyst dosage, initial concentration of phenol, temperature, pH and change in pH were systematically investigated. The highest phenol degradation rate was obtained at pH 9.0, temperature 60 C and catalyst dose of 2 g L¡1 with higher mineralization efficiency (in terms of TOC reduction). Experimental results showed that under optimized conditions the phenol removal efficiency was 98% and 100% for the TiO2/UV and TiO2/UV/H2O2 system, respectively. No significant effect on addition of chloride and metal ions was observed. Photodegradation of phenol followed first-order kinetics. To test whether the phenol removal was possible for wastewater using a TiO2/UV system, the degradation study was conducted with the real obtained wastewater. The removal of phenol from obtained wastewater and the synthetic wastewater containing phenol was comparable. The TiO2/UV system developed here is expected to be useful for the treatment of wastewater containing phenol. Keywords: Photocatalytic degradation, TiO2/UV, phenol, H2O2, pH, TOC, wastewater.
Introduction The frequent occurrence of phenolic compounds in wastewater has heightened concerns over public health.[1] Various chemical processes and agricultural industries generate large quantities of phenol and its intermediate compounds in their waste solutions. Phenol and its derivatives are stable, have weak acidic properties, and they are bio-accumulated. Therefore remain present in the environment. Phenols are of greater research interests due to their toxicity, carcinogenicity, teratogenicity and mutagenicity to humans, even at low concentrations.[2] These findings lead to further research on the removal of phenolic compounds from aqueous solution to minimize their accumulation. Existing conventional methods have inherent drawbacks of being economically unfeasible. They are also unable to completely eradicate remove phenol and/or their metabolites and they also generate wastes during the treatment operation, which requires additional removal steps and related increased costs.[3]
Address correspondence to Dong-Su Kim, Department of Environmental Science and Engineering, Ewha Womans University, Daehyundong 11-1, Seodaemungu, Seoul 120-750, Republic of Korea; E-mail: [email protected] Received April 2, 2014.
In recent years, advanced oxidation processes (AOP), which involve the generation of highly reactive hydroxyl radical (OH), have emerged as a promising water and wastewater treatment technology for the degradation or mineralization of a wide range of organic contaminants.[3,4] Under certain conditions concentrations above 2000–4000 mg L¡1, phenol can be economically recovered from wastewaters. However below these concentrations, phenol destruction is the best method for treating phenolic wastes.[5] A photocatalytic degradation process (PCD) is gaining importance for wastewaters containing small amounts of organic contaminants because of advantages such as being inexpensive, having a higher mineralization efficiency, presenting no waste disposal problems and only requiring mild temperature and pressure conditions by which the process becomes practically applicable.[6,7] The photoactivated reactions are characterized by the combined action of semiconductor photocatalyst, an energetic radiation source and an oxidizing agent governs the destruction of recalcitrant organics into relatively innocuous end products such as CO2 and H2O.[7] Generally the ideal photocatalyst should be chemically and biologically inert, photoactive, photostable, non-toxic and should be excited with visible and near/or ultraviolet (UV) light. Among many semiconductor photocatalysts for the degradation of organic contaminants there is a general consensus among researchers that TiO2 is superior due to its high
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Photocatalytic degradation of phenolic contaminants in wastewater activity, large stability to light illumination, low price, non-toxicity and commercially availability in different forms and particle characteristics.[8] An actively researched green chemistry approach to treat recalcitrant and harmful organic priority pollutants is titanium dioxide (TiO2)-mediated photocatalysis. Many organic contaminants can be decomposed or mineralized in aqueous solution by using TiO2 powder illuminated with UV light.[9,10] Some investigators reported TiO2 having higher photocatalytic efficiency relative to other photocatalysts (a-Fe2O3, ZrO2, CdS, WO3 and SnO2).[11] Extensive studies on photocatalytic degradation of phenols and chlorinated phenols in aerated aqueous suspensions of TiO2 upon illumination with near-UV light have been reported.[9,10,12] Okomoto et al.[13] observed greater photocatalytic activity for TiO2 compared to CdS catalyst for the decomposition of phenol as the target organic species. In addition, TiO2 was found to be useful for the elimination of microorganisms and for odor control, as has been reported. [14] Degussa TiO2 P-25 powder commercially used as a standard material in the photocatalytic oxidation process was chosen because it hasa relatively large surface area (55 m2 g¡1), which provides the evidence of a synergistic effect between contacting anatase and rutile particles.[15,16] Moreover, it was observed that photocatalytic degradation of phenolic compounds are largely dependent on various operational parameters such as type of catalyst and composition, solution pH, organic substrate concentration, light intensity, catalyst loading, ionic composition of wastewater, types of solvent, oxidant concentration etc.[11] Understanding the influence of these parameters on the photocatalytic degradation efficiency is of paramount importance for the design. From operational points of view, and to make the process more efficient, these parameters were included in this study. In this work the photodegradation of phenol was studied using TiO2, UV, and TiO2/UV and TiO2/UV/H2O2 systems. Systematic experiments were conducted to investigate the effects of operating parameters, such as initial concentration of pollutant, effect of pH and temperature and role of catalyst on phenol degradation using TiO2/ UV system. The extent of mineralization during degradation of phenol and its intermediates was also determined using TOC measurement. Phenol treatment efficiency was also studied for actual wastewater containing phenol using the TiO2/UV system.
Materials and methods Materials and reagents Titanium dioxide (Degussa P-25, ca. 70% anatase and 30% rutile) supplied from Degussa, Germany, has a surface area of 55 m2 g¡1 and an average particle size of 20–50 nm; it
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used as received without any pretreatment. Phenol (99.5% purity) was purchased from Duksan Pure Chemicals Co., Korea) in the crystalline state. A 200-W mercury lamp (G6T5, Japan Sankyo Denki Germicidal Lamp) with a spectral irradiance of the UV lamp ranges from 228 to 420 nm at a distance of 1 m from the light source. The light intensity of UV lamp used for the experiments was recorded with an UV/Visible spectrophotometer (Jasco, V550, Japan). All other chemicals used in this work were of the highest purity available and of reagent-grade quality.
Equipment Photodegradation experiments were conducted in a batch photoreactor contained in a cylindrical glass cell, covered with aluminum foil of 1.0 L capacity (100 mm diameter, 210 mm height) for protection against light interference. In the center of the reactor a UV lamp was placed in a quartz tube of 40 mm diameter and then immersed in the photoreactor cell. Titanium dioxide (TiO2) as a semiconductor material (Me Tops, Model- MS-300, Korea) was suspended in the phenol solution. Further the suspended solution was kept uniform by agitation with a magnetic stirrer.
Procedure Influencing factors including photocatalyst dosage (0.5–6.0 g L¡1), initial concentration of phenol (i.e., synthetic wastewater (SW); 5–30 mg L¡1), effect of initial pH (3.0–11), effect of temperature (25–60 C), effect of H2O2 (0.08 mM to 0.3 mM), addition of chloride ions (1mM each) and effect of chromium (Cr) ion (0.07 mM to 0.2 mM) on photocatalytical degradation of phenol using TiO2/UV system were studied. Solutions were irradiated by UV light. Experiments were conducted for 300 min, and the liquid samples (5 mL) were withdrawn at preset time intervals (every 60 min). Determination of phenol concentration was performed on an HPLC (Waters, 515, USA) equipped with UV detector and column of Xerra RP 18.5 um using a dual lambda absorbance detector. A mixture of acetonitrile and deionized water (80:20), flowing at a rate of 1.0 mL min¡1, was used as the mobile phase. Analytical samples were subjected to filtration through a 0.22-um syringe filter (Millipore, cellulose acetate membrane) to remove fine particles as well as TiO2 powders before HPLC analysis of phenol concentration. Each experiment was duplicated at least under identical conditions. The UV spectra of phenol solutions before and after irradiation were measured using a Jasco V-550 (Japan) spectrophotometer at 269 nm. A digital pH meter (Orion model 420A, USA) was used to measure the pH of the solution. The total organic carbon (TOC) of the sample was analyzed with a Shimadzu TOC-5000A analyzer by
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applying standard procedure.[17] The residual TOC of the samples were taken before and after photocatalytic degradation in which the irradiation time was adjusted at 1.5 and 2.5 min using oxygen as a carrier gas (0.95–1.00 bar). The TOC measurement was conducted under a maximum temperature of 850 C and a flow rate of 200 mL min¡1.
The phenol wastewater used in this study was obtained from a local institute located nearby the Seoul area, South Korea. The photocatalytic degradation test was carried out for real wastewater and finally the result was compared with that for synthetic wastewater. The phenol content in the real wastewater was between 108–132 ppm, pH 8.4 § 0.1, and SS were in the range of 62–98 ppm. Real wastewater was filtered first to remove the suspended solids, and then the concentration of phenol and the pH (9.0) was adjusted by considering the experimental conditions for synthetic wastewater. The photocatalytic degradation experiments using TiO2/UV system were conducted following the same procedure mentioned previously.
Results and discussion Efficiency of the photocatalyst The phenol degradation efficiency investigated by running the experiment in various conditions such as direct photolysis of phenol by irradiation with (UV; 200 W) exposed to the solution, in the presence of photocatalyst adsorption (TiO2) only, and the effect in combination of both TiO2 and UV sources. The result of the photocatalysis for 200 min for phenol is shown in Figure 1. The figure compares the photodegradation of phenol (10 mg L¡1) in aqueous solution under various reaction conditions. The
Effect of TiO2 dosage on phenol photodegradation The influence of catalyst dosage is an important parameter in suspended photocatalytic degradation studies. The effect of dosage of photocatalyst, various amounts of TiO2 (0.5–6.0 g L¡1) were suspended in phenol (10 mg L¡1) solution was investigated using HPLC after 100 min is shown in Figure 2. It was found that UV irradiation in the absence of TiO2 catalyst (i.e., direct photolysis) shows 140
UV (control) -1 6gL -1 4gL -1 2gL -1 1gL -1 0.5 g L
TiO2/ UV
120
120
UV TiO2
100
100
C/Co (%)
C/Co(%)
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Real wastewater study
results of the TiO2 (2 g L¡1) adsorption experiment leads to an extremely poor phenol removal (17%), yet the efficiency by direct photolysis of phenol with 200-W UV alone showed moderate phenol removal (44%). On the other hand, when the reaction was run in the presence of TiO2/UV, the process of photocatalytic degradation improved, showing phenol removal up to 98% after 100 min and after up to 200 min, complete degradation was observed. These results are similar with the studies using 400-W UV and with 1 g L¡1 TiO2 (56%) for phenol degradation.[18] The combination of two systems (TiO2/ UV) significantly enhanced the photocatalytic oxidation of phenol in comparison with that of photocatalyst TiO2 alone and photolysis using UV alone. From the principle mechanism of photocatalytic oxidation, the increased efficiency may be due to the holes (hC) that are formed in the valence band (VB), and electrons (e¡) can invoke strong oxidative and reductive environments. Hydroxyl (OH) and superoxide (O2) radicals are generated in solution, which effectively degrades the phenol. Thus from the results, oxidation of phenol by combination of photocatalytic (TiO2/UV) reaction was much stronger than photodegradation only or adsorption only. Photocatalytic degradation lead to higher degradation after 200 min of reaction, so further studies were carried out using the TiO2/UV combined system.
80 60
80 60 40
40
20
20
0
0 0
20
40
60
80
100
120
140
160
180
200
Time (min)
Fig. 1. Degradation of phenol (10 mg L¡1) during photocatalysis (TiO2/UV), photolysis (UV) and TiO2 (2 g L¡1) experiment.
0
20
40
60
80
100
Time (min)
Fig. 2. Effect of the dosage of TiO2 catalyst on phenol removal using TiO2/UV system (phenol concentration; 10 mg L¡1).
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120 (a)
(b)
5 mg L-1 10 mg L-1 30 mg L-1
5 4
ln (C/Co)
100 80 C/Co (%)
extremely low phenol removal (44%). In Figure 2, it is clear that when TiO2 dosage increased from 0.5 to 2 g L¡1, a high degradation rate of phenol (up to 98%) was achieved after 100 min. However continuing to increase the photocatalyst dosage (4 and 6 g L¡1) led to the increase of solution turbidity, and the photocatalytic degradation effect became worse, resulting in decreased efficiency value down to 25%. Similar performance was observed in the case of Pt/TiO2 dosing on the photocatalytic degradation of phenol.[5] Interaction between the OH radical and phenol becomes enhanced by TiO2, so a sufficient amount of catalyst increases the formation of electrons or hole pairs, thereby generating OH radicals to increase photodegradation.[19] Also, an increased dose concentration resulted in the blockage of light penetration on the surface (shielding effect), which reduced the reaction rate. The negative effect of the catalyst concentration on the degradation efficiency may be due to the aggregation/agglomeration of catalyst that lowers the total number of active sites on the surface.[20] Therefore, increasing photocatalyst dosage properly could enhance the photocatalytic degradation effect because of the greater amount of active substance available to treat wastewater. The optimal dosage of 2 g L¡1 catalyst gave a better performance, and the catalyst dose of 2 g L¡1 was used for further studies.
3 2 1
60
0 0
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Time (min)
40 20 0 0
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Time (min)
120 (c)
5 mg L-1 10 mg L-1
100
TOC reduction (%)
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Photocatalytic degradation of phenolic contaminants in wastewater
30 mg L-1
80 60 40 20
Effect of the initial phenol concentration on photodegradation
0 0
Results of photodegradation of phenol at different initial concentrations (5–30 mg L¡1) using TiO2/UV are shown in Figure 3. It can be seen that when the initial phenol concentration was 5 mg L¡1 and 10 mg L¡1, it showed 100% and 98% phenol removal efficiency within 100 min, along with an increase in photocatalytic oxidation, yet the efficiency was drastically decreased at high concentration of 30 mg L¡1 (45%) (Fig. 3a). It was observed that at the equilibrium adsorption, the active catalyst sites are loaded with phenolic species. Hence, competitive adsorption of OH¡ on the same sites is less likely, which results in a decrease in the amount of OH¡ and O2¡ on the catalyst surface results, or reduced degradation efficiency.[20] The Langmuir–Hinshelwood model is usually used to explain the kinetics of photocatalytic reactions of aqueous solution containing organics. It is mainly to study the degradation rate and organic concentration. The rate of decrease of phenol concentration in solution is assumed to follow a first-order reaction as: [21] dc D kc ¡ dt
(1)
where C is the concentration of phenol in solution at a given time, t, and k is the reaction rate constant. Further, the integration to the preceding equation yields an
50
100
150
200
250
300
Time (min)
Fig. 3. (a) Effect of the initial concentration (5 mg L¡1 to 30 mg L¡1) on phenol removal; (b) Linear plot of time versus ln (C/ C0); (c) TOC removal depending on the initial concentration using TiO2/UV system.
exponential concentration profile with time, C D C0 exp. ¡ kt/
(2)
where Co, is the initial phenol concentration in solution. ln
C D ¡ kt C0
(3)
A plot of ln (C/C0) versus reaction time t yields a straight line, and the slope is the rate constant. The experimental data in Figure 3b was found to be fitted to the preceding equation and the value of the reaction rate constant, k with regression coefficient for phenol at different concentrations, as shown in Table 1. At the initial concentration of phenol (5 mg L¡1), the removal rate increased, but as
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Table 1. Effect of initial concentration of phenol on photocatalytic degradation of phenol using TiO2/UV system. Co (mg L¡1)
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5 10 30
Degradation ratio after 100 min (%)
K (min )
Correlation coefficient (R2)
100 98 45
0.0594 0.0269 0.0065
0.9701 0.9352 0.92661
¡1
the concentration increased (30 mg L¡1) the removal rate was reduced. This could be because as the initial concentration of phenol increases, a large number of phenol molecules is adsorbed on the surface of TiO2, which reduces the OH radical formation, since very few active sites remained for the adsorption of hydroxyl ions and OH radicals. From Table 1 it can be seen that the rate constant and the R2 value decreased with increasing phenol concentration. This could be due to the intermediates generated during the photocatalytic process that affect the overall phenol degradation rate. The attack of these radicals on organic compounds decreased, thereby decreasing photodegradation efficiency. The total organic carbon remaining in the solution after photocatalytic degradation of phenol by TiO2/UV for different phenol initial concentrations (5, 10 and 30 mg L¡1) is illustrated in Figure 3c. The amount of TOC removal is found to be best at lower phenol concentrations. As the concentration increased, the decrease in TOC removal showedpoor performance, which may be due to poorly formed OH radicals because of saturation. TOC removal is greatly accelerated with 90%, 83% and 35% for 5, 10 and 30 mg L¡1, respectively, of phenol concentration after 300 min of reaction. Thus, combining TiO2/UV enhances photodegradation of organic pollutants.
Change in pH during photocatalytic degradation of phenol Effects of changes in solution pH with time on the photocatalytic degradation of phenol (10 mg L¡1) has been studied (data not shown). In this experiment, the pH of the distilled water was 6.87; and on addition of phenol solution the pH was 5.62. But when TiO2 (2 g L¡1) was added the pH dropped to 4.32 after 5 h. Early in the reaction, pH was higher than 6.3, which is the pzc (point of zero charge) of TiO2, so that concentration of OH¡ ion is decreased by adsorption of OH¡ ions on the surface of the catalyst. As a result then, pH decreases. Additional decrease in pH after some reaction is due to OH¡ ions that continuously decrease from the oxidation reaction on the catalyst surface; byproducts are formed by degradation of phenol. Specifically, phenol is an acid that provides hydrogen ions in
a numeric field. The acid-base reaction is a very rapid reaction, and it can be represented by Eq. 4: ½H C ½P ¡ D Ka ½HP
(4)
where [HC] is hydrogen ion (mol L¡1), [PC] is phenol (mol L¡1), [HP] is; not dissociated phenol (mol L¡1), and Ka is acidity constant. Concentration of hydrogen ions in water ranges from approximately 10¡4 to 10¡10 mol L¡1.[22] Concentration of hydrogen ions is mainly represented by pH. When pH of the solution is the same as the pKa value of the acid, 50% of the acid provides hydrogen ions in solution in the form of anions. When pH value is greater than the pKa value, organic acid provides all the hydrogen ions which exist in the form of anions. Thus phenol is weakly acidic and so seems to decrease the pH of the solution. Effect of pH The hydrogen ion concentration of the aqueous solution largely influences the behavior of toxic organic pollutants. The surface of TiO2 photocatalyst used in the photocatalytic oxidation reaction depends on the pH of the aqueous solution. Generally, in TiO2 it was observed that it shows a pzc, which is not charged with electrical polarity when the pH of the aqueous solution is 6.3. Therefore, when the pH of the solution is low (acidic), HC ions are adsorbed on the catalyst surface and have the properties of the anode. Conversely, at high pH (basic), OH¡ ions adsorbed on the surface become cathodic.[23,24] The pzc of Degussa P25 that was used in this study is also of pH 6.3. Thus it can be seen that where the pH > 6.3, the surface of the TiO2 interferes with the activity of OH¡ radicals, extruding the negative ions, which become significantly negative charge. By contrast, in the case of pH < 6.3, to attack the negative ions to hydrate the surface of TiO2, organic matter attains a significantly positive charge.[12] TiIV ¡ OH C H C ! TiIV ¡ OH2
(5)
TiIV ¡ OH C OH ¡ ! TiIV ¡ O ¡ C H2 O
(6)
Thus, the pH of the solution changes the surface of TiO2 catalyst, which is dispersed in aqueous solution and is characteristic of adsorption and desorption, which influences the speed of production of OH radical as a result of the overall reaction rate. Figure 4a shows the effect of different solution pHs with time on the photocatalytic degradation of phenol over TiO2 with UV light. The phenol removal was gradually increased, with time up to 100 min for a wide range of pH values. It was found that increase in solution pH increases the percentage of degradation The maximum percentage
Photocatalytic degradation of phenolic contaminants in wastewater 100
(a)
pH 3 pH 7 pH 9 pH 11
C/Co (%)
80
60
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20
0 0
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120
Degradation Efficiency (%)
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Time (min) (b) 10 min 50 min 100 min
100
60 40 20 0 4
The lowering of the degradation percentage with increasing pH can be described by the amphoteric nature of TiO2 in aqueous solution. Figure 4b shows the effects of phenol removal when TiO2 2 g L¡1 in 10 mg L¡1 phenol. From the results, pH 9 shows the best effects; however, pH 11 shows the opposite result, so we assume it is because of the OH group. Actually, the surface of the catalyst shows a negative change at high pH from the repulsive forces between phenol ions, which dissociate as anions in the solution and are difficult to adsorb on the active site of surface of TiO2, thereby decreasing the reaction rate. Also hydroxyl ion creates a strong oxidative OH radical by capturing the hole (hC) formed on the surface of TiO2. The proper number of protons are needed in the photoreduction reaction using TiO2. When there are too many protons, TiO2 becomes acid, and so electron (e¡) holes will recombine rapidly.
Effect of temperature
80
2
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6
8
10
12
pH
Fig. 4. Degradation of phenol (10 mg L¡1) (a) depending on the initial pH; (b) degradation efficiency of phenol according to the initial pH (3 to 11) using TiO2/UV system.
degradation values of phenol were found to be 68.21%, 78.14%, 98.00%, and 80.42% with solution pH values of 3, 7, 9, and 11, respectively. At lower pH, decrease in phenol degradation may be due to the decrease in hydroxyl ions that are required to react with valence band hole to form OH radicals.[5] However, the degradation of phenol in solutions was enhanced in the alkaline medium rather than in neutral or acid conditions, and the optimum pH value was 9 with maximum removal of phenol (98%). Bahemamm et al.[25] showed that when the intensity of light is not strong, the change in pH strongly affected the functional groups of the TiO2 surface in the acidic region by the following equations:
The effects of temperature on photocatalytic degradation of phenol were investigated (Fig. 5). Although sometimes as the solution temperature increases the percent degradation of phenol is increased, this type of increase could be explained by the reverse reaction that hydrogen is oxidized into water by photocatalytic reaction, and also due to increase in the reaction rate taking place between the phenol molecules and the OH radicals. Matthews[26] reported that temperature increase enhances the reaction rate and consequently induces the decomposition reaction of organic matter. The performance was higher at higher temperature due to collision frequencies of phenol molecules.[27] In this study, temperature was regulated at 25, 40 and 60 C to study the effects of temperature on the reaction rate. The result of this study are shown in Figure 5. For the performance of phenol (10 mg L¡1), it was observed that almost complete decomposition occurs as temperature increased (Fig. 5a), so the efficiency of the reaction cannot be understood. Further the reaction applied for phenol (50 mg L¡1), due to higher concentration, the reaction was hard to observe clearly; however, it can be seen that as time passes, the efficiency in phenol remove decreased at higher concentrations (Fig. 5b). However the optimum temperature in our study was observed to be 60 C. This is because as the temperature increases, the adsorption rate constant or surface reaction rate on the catalyst surface increases.
Effect of H2O2 TiOH C H C $ TiOH2C .acidic condition/
(7)
TiOH C OH ¡ $ TiO ¡ C H2 O .alkaline condition/ (8)
An advanced oxidation process using UV/H2O2 is another way to degrade phenolic compounds in aqueous solutions.[28] The UV/H2O2 effect results in formation of OH radicals, which accelerate the oxidative degradation of the phenolic
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100
(a)
80
120 o 25 C o 40 C o 60 C
TiO2/UV+0.3 mM H2O2 TiO2/UV+ 0.15 mM H2O2 TiO2/UV+ 0.1 mM H2O2 TiO2/UV+0.08 mM H2O2
100
UV+0.08 mM H2O2 UV
C/Co (%)
C/C0(%)
80 60
40
60 40 20
20
0 0
0 20
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Time (min)
Time (min) 100
(b) 80
C/Co (%)
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0
o 25 C o 40 C o 60 C
60
Fig. 6. Degradation of phenol depending on the various H2O2 (0.08 to 0.3 mM) using TiO2/UV system. Reaction conditions: (pH D 9, TiO2 D 2 g L¡1, phenol: 10 mg L¡1).
in the case of the degradation of 2-chlorophenol.[29,30] Equations for photocatalytic degradation involving H2O2 in the H2O2/UV system are presented as follows: [31]
40
H2 O2 C hv ! 2OH
(9)
20
h C C OH ¡ ! OH ¡
(10)
e ¡ C O2 ! O2¡
0 0
100
200
300
400
500
¡
e C H2 O2 ! OH C OH
(11) ¡
(12)
Time (min)
Fig. 5. Degradation of phenol (a) depending on the temperature at 10 mg L¡1 phenol and (b) at 50 mg L¡1phenol using TiO2/ UV system.
compounds in water. The problem of high cost of H2O2 and the low oxidation rate of TiO2 can be solved by combining both systems. The effect of H2O2 addition on phenol photodecomposition in the presence of TiO2/UV was also investigated. Figure 6 compares the photodegradation of 10 mg L¡1 phenol in aqueous solutions under various reaction conditions. Photocatalytic degradation of phenol by TiO2/UV (TiO2; 2 g L¡1; 200 W) showed 98% of phenol removal. When the reaction was carried out in presence of TiO2/ UV/H2O2 the performance was better than the TiO2/UV. So, the experiments were carried out in various concentration of H2O2 addition to the reaction for TiO2/UV/H2O2. Thus for the system containing TiO2/UV/H2O2, enhancement in the phenol removal was observed with increase in the H2O2 concentration. The phenol removal improved and complete degradation was observed at higher concentration of H2O2 (0.3 mM). The removal efficiency increased up to 100% when H2O2 was added to the UV/ TiO2 system. A similar type of performance was also seen
Phenolic compound C OH ! H2 O C R
(13)
! further oxidation Photolysis by H2O2/UV generates OH radicals from Eq. 9 in presence of UV light promoting the rate of reaction. It was reported that H2O2 is a stronger electron acceptor than oxygen and thus prevents the recombination between holes/electrons, which means that the OH radical generated by H2O2 reacts with holes, decreasing the chances of holes/electron recombination, as shown in Eq. 12. However by producing electrons which is directly used in oxidation reaction of phenol, the rate of phenol photodegradation increased significantly in the TiO2/UV/H2O2 system. In the case of a real wastewater study using a TiO2/UV system and TiO2/UV/H2O2 (0.3 mM) system shows 96.7% and 98.4% after 200 min of reaction, respectively (Table 2). However, the performance of a photocatalytic system using TiO2/UV as a photocatalyst in presence/ absence of H2O2 with our system for phenol degradation has been systematically compared with ones reported earlier, indicating the photocatalyst tested in this study showed fairly good performance for significant phenol removal (Table 3).
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Table 2. Performance of TiO2/UV system in presence of different ions for the photocatalytic degradation of phenol containing synthetic wastewater (SW) and real wastewater (RW). Synthetic wastewater (SW) Parameters studied TiO2/UV TiO2/UV/H2O2 (0.3 mM) TiO2/UV/NaCl (1 mM) TiO2/UV/CHCl3 (1 mM) TiO2/UV/K2Cr2O7 (0.07 mM) TiO2/UV/K2Cr2O7 (0.2 mM)
Time (min)
Degradation (%)
Time (min)
Degradation (%)
100 100 300 200 300 300
98 § 0.74 100 § 0.82 100 § 0.72 100 § 0.83 80 § 0.64 66 § 0.48
200 200 300 300 300 300
96.7 § 0.76 98.4 § 0.70 98.1 § 0.75 97.6 § 0.82 78.2 § 0.70 64.8 § 0.54
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Real wastewater (RW)
During photocatalytic reactions, ferric ion acts as electron acceptors that prevent the recombination of holes with electrons efficiently, thereby enhancing photocatalytic oxidation of phenol. Chloride ion is also a substance that acts as an electron donor like HO2 and H2O2, but Cl¡ sometimes hinders and reduces catalytic activation by adsorption to the active sites. Detailed descriptions for chloride ions that scavenge the photonproduced holes and OH radicals more effectively have been reported.[32] The effects of different chloride ions on photocatalytic decomposition of phenol were studied in synthetic wastewater (SW) and real wastewater (RW) (Figs. 7a and b). Different chloride sources such and NaCl and CHCl3 (1 mM each) were used to study their effects, and the results showed very similar trends with that of the TiO2/ UV system. Addition of chloride ion shows no significant effect on the observed photocatalytic decomposition of phenol. It has been reported that at higher pH the negatively charged catalyst surface repelled the coming of chloride ions to the surface and restricted their adsorption without influencing the decomposition of organic compound.[33] The other reason could be that during reaction; chloride ions along with free radicals decreased the chances of the reactive reactant molecule and free radical and
thus lowered the performance in presence of chloride ions. Literature studies also observed negative effect of chloride ions for the degradation of 4-chlorophenol and phenol.[34,35] The foregoing reasons suggest that chloride ions in the aqueous solution could act as electron scavengers, competing with molecular oxygen for the active site, thereby lowering the reaction rate.
Effect of chromium ion In this study, as with the effects of chloride ions, of the presence of selected chromium Cr (VI) (Cr2O72¡) metal ion was also examined. It has been reported that metal ions such as transition metal ions and noble metal ions deposited on a photocatalyst can increase the photocatalytic efficiency according to the redox process.[32] Generally it is considered that the metal sites act as trapping sites and function as electron acceptor and improves the photocatalytic degradation activity by impeding the recombination of holes with electrons. Figure 8a and b shows the variation in the concentration of chromium (0.07 and 0.2 mM K2Cr2O7) according to the reaction time in the presence and absence of Cr for SW and RW using a TiO2/UV system. Similar to the results of chloride, almost no induction in the efficiency of photodegradation of phenol was observed by the addition of Cr ion. The reason may be due to the
Table 3. Comparison of phenol degradation study by using TiO2 photocatalyst with literature under various conditions. TiO2 g L¡1
UV
H2O2 300 mM 10 mM 1.77 mM — —
7.5
300 W 17 W/m2 400 W 400W 260 W/m2 22.5 W/m2
2.0
200 W
0.3 mM
0.1 1.0 1.0 1.0 0.5*
*Pt/TiO2. nd: not determined.
—
Type of phenolic waste
Concentration (mg L¡1)
pH /Temp ( C)/Time (min)
Removal efficiency (%)
Phenol Phenol Phenol Phenol Phenol and 2-cholrophenol 2-chloro and 2-nitrophenol Phenol
100 10 50 10 50
8 /nd/ 60 7 / 25 / 120 6.8 / 25 / 180 6.7–7.2 /32/ 300 3 /25 / 180
[36] [37] [29] [38] [5]
10
3 /25 / 140 min
99.2 97.0 94.0 99.0 87.0 100 95.0
10
9 /60 / 100
100
This study
Reference
[33]
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120 (a)
TiO2/UV + S.W. TiO2/UV +1 mM NaCl+ S.W.
100
0
C/C (%)
0
C/C (%)
TiO2/UV +S.W.+ 0.2 mM K2Cr2O7
80
60
60
40
40
20
20 0 120
0 120
(b)
TiO2/UV + R.W.
(b)
TiO2/UV + R.W.+ 0.07 mM K2Cr2O7
TiO2/UV + S.W. TiO2/UV + 1mM CHCl3+S.W.
TiO2/UV + R.W.+ 0.2 mM K2Cr2O7
100
TiO2/UV +R.W. TiO2/UV + 1mM CHCl3+R.W.
C/C (%)
100
0
80
0
C/C (%)
TiO2/UV + S.W. TiO2/UV + S.W.+ 0.07 mM K2Cr2O7
100
TiO2/UV + R.W. TiO2/UV+1 mM NaCl+R.W.
80
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(a)
60
80 60 40
40 20
20 0 0
0 0
50
100
150
200
250
Time (min)
Fig. 7. Effect of chlorides on TiO2/UV system for the removal of phenol in synthetic (SW) and real wastewater (RW) (a) NaCl (1 mM); (b) CHCl3 (1 mM). Reaction conditions: (pH D 9, TiO2 D 2 g L¡1, phenol: 10 mg L¡1).
addition of smaller amounts of Cr in aqueous solution, and another may be the difficulty of reduction of Cr (VI) to Cr (III) because of its highly reactive oxidation/reduction potentials under given conditions. Thus the photocatalytic activity was not significantly affected by the addition of chloride and metal ions that may have inhibited the formation of OH¡, preventing phenolic molecules from reaching the surface active site. The TiO2 surface is negatively charged and repulsive forces will lead to decreased adsorption at high pH.
Removal of phenol from real wastewater by TiO2/UV system Experiments were carried out to find the effectiveness of the TiO2/UV system for removing phenol from real wastewater. Experiments were conducted partly to examine the possible differences in degradation features for real wastewater compared to the results from a synthetically prepared phenol aqueous solution. It was observed that when the TiO2/ UV system was used for photocatalytic degradation, the
50
100
150
200
250
300
Time (min)
300
Fig. 8. Effect of chromium (0.07 mM K2Cr2O7 and 0.2 mM K2Cr2O7) ions on TiO2/UV system for the removal of phenol in (a) synthetic wastewater (SW) and (b) real wastewater (RW). Reaction conditions: (pH D 9, Phenol: 10 mg L¡1; TiO2: 2 g L¡1).
treatment efficiency for real wastewater was found to be lower (96.7%) compared to artificial wastewater (98%) after 200 min. The real wastewater employed in the experiments was thought to contain some other organic substances, which may have led to the decrease in the degradation of phenol efficiency. This was also probably due to the interfering effects of the coexisting substances on the degradation of phenol. The addition of H2O2, chloride and chromium ions to increase the treatment efficiency was also studied. Treatment efficiency excluding H2O2 showed no significant effect on the treatment efficiency in the presence of chloride ions and metal ions (Table 2). The removal and or treatment efficiency of phenol synthetic wastewater was found to be very comparable with that obtained phenol from wastewater samples (Table 2).
Conclusion Efficiency of photocatalytic degradation of phenol by using a TiO2/UV system in aqueous solution was studied.
Photocatalytic degradation of phenolic contaminants in wastewater
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The TiO2/UV system showed the highest efficiency towards phenol degradation compared to catalyst alone or direct photolysis (UV alone), at similar experimental conditions. Significant removal of phenol (10 mg L¡1) was observed at photocatalyst (2 g L¡1), at alkaline pH 9.0 and temperature at 60 C within 100 min with higher mineralization in terms of TOC. The addition of chloride ions and metal ions showed no significant response for phenol degradation. Addition of H2O2 improved the degradation efficiency, and complete removal of phenol was observed. The TiO2/UV system used to evaluated for phenol degradation of systems containing real wastewater exhibited significant degradation performance and was determined to have strong practical applications.
[11]
[12] [13]
[14]
[15]
[16]
Funding [17]
The project was supported by the Fundamental Technology R & D Program for Society of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant number: 2013 M3C8A3078596 and 2009-0083527).
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[31] Chiou, C.H.; Wu, C.Y.; Juang, R.S. Influence of operating parameters on photocatalytic degradation of phenol in UV/TiO2 process. Chem. Eng. J. 2008, 139, 322–329. [32] Thennarasu, G.; Sivasamy, A. Metal ion doped semiconductor metal oxide nanosphere particles prepared by soft chemical method and its visible light photocatalytic activity in degradation of phenol. Powder Technol. 2013, 250, 1–12. [33] Wang, K.H.; Hsiehb, Y.H.; Choub, M.Y.; Chang, C.Y. Photocatalytic degradation of 2-chloro and 2-nitrophenol by titanium dioxide suspensions in aqueous solution. Appl. Catal. B: Environ. 1999, 21, 1–8. [34] Alhakimi, G.; Gebril, S.; Studnicki, H. Comparative photocatalytic degradation using natural and artificial UV-light of 4-chlorophenol as a representative compound in refinery
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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20
Influence of parameters on the photocatalytic degradation of phenolic contaminants in wastewater using TiO2/UV system a
a
a
a
Rijuta G. Saratale , Hyun S. Noh , Ji Y. Song & Dong S. Kim a
Department of Environmental Science and Engineering, Ewha Womans University, Seoul, Republic of Korea Published online: 19 Aug 2014.
To cite this article: Rijuta G. Saratale, Hyun S. Noh, Ji Y. Song & Dong S. Kim (2014) Influence of parameters on the photocatalytic degradation of phenolic contaminants in wastewater using TiO2/UV system, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:13, 1542-1552, DOI: 10.1080/10934529.2014.938532 To link to this article: http://dx.doi.org/10.1080/10934529.2014.938532
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Journal of Environmental Science and Health, Part A (2014) 49, 1542–1552 Copyright © Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2014.938532
Influence of parameters on the photocatalytic degradation of phenolic contaminants in wastewater using TiO2/UV system RIJUTA G. SARATALE, HYUN S. NOH, JI Y. SONG and DONG S. KIM
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Department of Environmental Science and Engineering, Ewha Womans University, Seoul, Republic of Korea
The photocatalytic degradation of phenol in aqueous suspension using commercial TiO2 powder (Degussa P-25) irradiated with UV light was investigated. Photodegradation was compared using a photocatalyst (TiO2 alone), direct photolysis (UV alone) and TiO2/ UV in a single batch reactor with mercury lamp irradiation. The study focused on the influence of various operating parameters on phenol treatment efficiency, including catalyst dosage, initial concentration of phenol, temperature, pH and change in pH were systematically investigated. The highest phenol degradation rate was obtained at pH 9.0, temperature 60 C and catalyst dose of 2 g L¡1 with higher mineralization efficiency (in terms of TOC reduction). Experimental results showed that under optimized conditions the phenol removal efficiency was 98% and 100% for the TiO2/UV and TiO2/UV/H2O2 system, respectively. No significant effect on addition of chloride and metal ions was observed. Photodegradation of phenol followed first-order kinetics. To test whether the phenol removal was possible for wastewater using a TiO2/UV system, the degradation study was conducted with the real obtained wastewater. The removal of phenol from obtained wastewater and the synthetic wastewater containing phenol was comparable. The TiO2/UV system developed here is expected to be useful for the treatment of wastewater containing phenol. Keywords: Photocatalytic degradation, TiO2/UV, phenol, H2O2, pH, TOC, wastewater.
Introduction The frequent occurrence of phenolic compounds in wastewater has heightened concerns over public health.[1] Various chemical processes and agricultural industries generate large quantities of phenol and its intermediate compounds in their waste solutions. Phenol and its derivatives are stable, have weak acidic properties, and they are bio-accumulated. Therefore remain present in the environment. Phenols are of greater research interests due to their toxicity, carcinogenicity, teratogenicity and mutagenicity to humans, even at low concentrations.[2] These findings lead to further research on the removal of phenolic compounds from aqueous solution to minimize their accumulation. Existing conventional methods have inherent drawbacks of being economically unfeasible. They are also unable to completely eradicate remove phenol and/or their metabolites and they also generate wastes during the treatment operation, which requires additional removal steps and related increased costs.[3]
Address correspondence to Dong-Su Kim, Department of Environmental Science and Engineering, Ewha Womans University, Daehyundong 11-1, Seodaemungu, Seoul 120-750, Republic of Korea; E-mail: [email protected] Received April 2, 2014.
In recent years, advanced oxidation processes (AOP), which involve the generation of highly reactive hydroxyl radical (OH), have emerged as a promising water and wastewater treatment technology for the degradation or mineralization of a wide range of organic contaminants.[3,4] Under certain conditions concentrations above 2000–4000 mg L¡1, phenol can be economically recovered from wastewaters. However below these concentrations, phenol destruction is the best method for treating phenolic wastes.[5] A photocatalytic degradation process (PCD) is gaining importance for wastewaters containing small amounts of organic contaminants because of advantages such as being inexpensive, having a higher mineralization efficiency, presenting no waste disposal problems and only requiring mild temperature and pressure conditions by which the process becomes practically applicable.[6,7] The photoactivated reactions are characterized by the combined action of semiconductor photocatalyst, an energetic radiation source and an oxidizing agent governs the destruction of recalcitrant organics into relatively innocuous end products such as CO2 and H2O.[7] Generally the ideal photocatalyst should be chemically and biologically inert, photoactive, photostable, non-toxic and should be excited with visible and near/or ultraviolet (UV) light. Among many semiconductor photocatalysts for the degradation of organic contaminants there is a general consensus among researchers that TiO2 is superior due to its high
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Photocatalytic degradation of phenolic contaminants in wastewater activity, large stability to light illumination, low price, non-toxicity and commercially availability in different forms and particle characteristics.[8] An actively researched green chemistry approach to treat recalcitrant and harmful organic priority pollutants is titanium dioxide (TiO2)-mediated photocatalysis. Many organic contaminants can be decomposed or mineralized in aqueous solution by using TiO2 powder illuminated with UV light.[9,10] Some investigators reported TiO2 having higher photocatalytic efficiency relative to other photocatalysts (a-Fe2O3, ZrO2, CdS, WO3 and SnO2).[11] Extensive studies on photocatalytic degradation of phenols and chlorinated phenols in aerated aqueous suspensions of TiO2 upon illumination with near-UV light have been reported.[9,10,12] Okomoto et al.[13] observed greater photocatalytic activity for TiO2 compared to CdS catalyst for the decomposition of phenol as the target organic species. In addition, TiO2 was found to be useful for the elimination of microorganisms and for odor control, as has been reported. [14] Degussa TiO2 P-25 powder commercially used as a standard material in the photocatalytic oxidation process was chosen because it hasa relatively large surface area (55 m2 g¡1), which provides the evidence of a synergistic effect between contacting anatase and rutile particles.[15,16] Moreover, it was observed that photocatalytic degradation of phenolic compounds are largely dependent on various operational parameters such as type of catalyst and composition, solution pH, organic substrate concentration, light intensity, catalyst loading, ionic composition of wastewater, types of solvent, oxidant concentration etc.[11] Understanding the influence of these parameters on the photocatalytic degradation efficiency is of paramount importance for the design. From operational points of view, and to make the process more efficient, these parameters were included in this study. In this work the photodegradation of phenol was studied using TiO2, UV, and TiO2/UV and TiO2/UV/H2O2 systems. Systematic experiments were conducted to investigate the effects of operating parameters, such as initial concentration of pollutant, effect of pH and temperature and role of catalyst on phenol degradation using TiO2/ UV system. The extent of mineralization during degradation of phenol and its intermediates was also determined using TOC measurement. Phenol treatment efficiency was also studied for actual wastewater containing phenol using the TiO2/UV system.
Materials and methods Materials and reagents Titanium dioxide (Degussa P-25, ca. 70% anatase and 30% rutile) supplied from Degussa, Germany, has a surface area of 55 m2 g¡1 and an average particle size of 20–50 nm; it
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used as received without any pretreatment. Phenol (99.5% purity) was purchased from Duksan Pure Chemicals Co., Korea) in the crystalline state. A 200-W mercury lamp (G6T5, Japan Sankyo Denki Germicidal Lamp) with a spectral irradiance of the UV lamp ranges from 228 to 420 nm at a distance of 1 m from the light source. The light intensity of UV lamp used for the experiments was recorded with an UV/Visible spectrophotometer (Jasco, V550, Japan). All other chemicals used in this work were of the highest purity available and of reagent-grade quality.
Equipment Photodegradation experiments were conducted in a batch photoreactor contained in a cylindrical glass cell, covered with aluminum foil of 1.0 L capacity (100 mm diameter, 210 mm height) for protection against light interference. In the center of the reactor a UV lamp was placed in a quartz tube of 40 mm diameter and then immersed in the photoreactor cell. Titanium dioxide (TiO2) as a semiconductor material (Me Tops, Model- MS-300, Korea) was suspended in the phenol solution. Further the suspended solution was kept uniform by agitation with a magnetic stirrer.
Procedure Influencing factors including photocatalyst dosage (0.5–6.0 g L¡1), initial concentration of phenol (i.e., synthetic wastewater (SW); 5–30 mg L¡1), effect of initial pH (3.0–11), effect of temperature (25–60 C), effect of H2O2 (0.08 mM to 0.3 mM), addition of chloride ions (1mM each) and effect of chromium (Cr) ion (0.07 mM to 0.2 mM) on photocatalytical degradation of phenol using TiO2/UV system were studied. Solutions were irradiated by UV light. Experiments were conducted for 300 min, and the liquid samples (5 mL) were withdrawn at preset time intervals (every 60 min). Determination of phenol concentration was performed on an HPLC (Waters, 515, USA) equipped with UV detector and column of Xerra RP 18.5 um using a dual lambda absorbance detector. A mixture of acetonitrile and deionized water (80:20), flowing at a rate of 1.0 mL min¡1, was used as the mobile phase. Analytical samples were subjected to filtration through a 0.22-um syringe filter (Millipore, cellulose acetate membrane) to remove fine particles as well as TiO2 powders before HPLC analysis of phenol concentration. Each experiment was duplicated at least under identical conditions. The UV spectra of phenol solutions before and after irradiation were measured using a Jasco V-550 (Japan) spectrophotometer at 269 nm. A digital pH meter (Orion model 420A, USA) was used to measure the pH of the solution. The total organic carbon (TOC) of the sample was analyzed with a Shimadzu TOC-5000A analyzer by
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applying standard procedure.[17] The residual TOC of the samples were taken before and after photocatalytic degradation in which the irradiation time was adjusted at 1.5 and 2.5 min using oxygen as a carrier gas (0.95–1.00 bar). The TOC measurement was conducted under a maximum temperature of 850 C and a flow rate of 200 mL min¡1.
The phenol wastewater used in this study was obtained from a local institute located nearby the Seoul area, South Korea. The photocatalytic degradation test was carried out for real wastewater and finally the result was compared with that for synthetic wastewater. The phenol content in the real wastewater was between 108–132 ppm, pH 8.4 § 0.1, and SS were in the range of 62–98 ppm. Real wastewater was filtered first to remove the suspended solids, and then the concentration of phenol and the pH (9.0) was adjusted by considering the experimental conditions for synthetic wastewater. The photocatalytic degradation experiments using TiO2/UV system were conducted following the same procedure mentioned previously.
Results and discussion Efficiency of the photocatalyst The phenol degradation efficiency investigated by running the experiment in various conditions such as direct photolysis of phenol by irradiation with (UV; 200 W) exposed to the solution, in the presence of photocatalyst adsorption (TiO2) only, and the effect in combination of both TiO2 and UV sources. The result of the photocatalysis for 200 min for phenol is shown in Figure 1. The figure compares the photodegradation of phenol (10 mg L¡1) in aqueous solution under various reaction conditions. The
Effect of TiO2 dosage on phenol photodegradation The influence of catalyst dosage is an important parameter in suspended photocatalytic degradation studies. The effect of dosage of photocatalyst, various amounts of TiO2 (0.5–6.0 g L¡1) were suspended in phenol (10 mg L¡1) solution was investigated using HPLC after 100 min is shown in Figure 2. It was found that UV irradiation in the absence of TiO2 catalyst (i.e., direct photolysis) shows 140
UV (control) -1 6gL -1 4gL -1 2gL -1 1gL -1 0.5 g L
TiO2/ UV
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120
UV TiO2
100
100
C/Co (%)
C/Co(%)
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Real wastewater study
results of the TiO2 (2 g L¡1) adsorption experiment leads to an extremely poor phenol removal (17%), yet the efficiency by direct photolysis of phenol with 200-W UV alone showed moderate phenol removal (44%). On the other hand, when the reaction was run in the presence of TiO2/UV, the process of photocatalytic degradation improved, showing phenol removal up to 98% after 100 min and after up to 200 min, complete degradation was observed. These results are similar with the studies using 400-W UV and with 1 g L¡1 TiO2 (56%) for phenol degradation.[18] The combination of two systems (TiO2/ UV) significantly enhanced the photocatalytic oxidation of phenol in comparison with that of photocatalyst TiO2 alone and photolysis using UV alone. From the principle mechanism of photocatalytic oxidation, the increased efficiency may be due to the holes (hC) that are formed in the valence band (VB), and electrons (e¡) can invoke strong oxidative and reductive environments. Hydroxyl (OH) and superoxide (O2) radicals are generated in solution, which effectively degrades the phenol. Thus from the results, oxidation of phenol by combination of photocatalytic (TiO2/UV) reaction was much stronger than photodegradation only or adsorption only. Photocatalytic degradation lead to higher degradation after 200 min of reaction, so further studies were carried out using the TiO2/UV combined system.
80 60
80 60 40
40
20
20
0
0 0
20
40
60
80
100
120
140
160
180
200
Time (min)
Fig. 1. Degradation of phenol (10 mg L¡1) during photocatalysis (TiO2/UV), photolysis (UV) and TiO2 (2 g L¡1) experiment.
0
20
40
60
80
100
Time (min)
Fig. 2. Effect of the dosage of TiO2 catalyst on phenol removal using TiO2/UV system (phenol concentration; 10 mg L¡1).
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(b)
5 mg L-1 10 mg L-1 30 mg L-1
5 4
ln (C/Co)
100 80 C/Co (%)
extremely low phenol removal (44%). In Figure 2, it is clear that when TiO2 dosage increased from 0.5 to 2 g L¡1, a high degradation rate of phenol (up to 98%) was achieved after 100 min. However continuing to increase the photocatalyst dosage (4 and 6 g L¡1) led to the increase of solution turbidity, and the photocatalytic degradation effect became worse, resulting in decreased efficiency value down to 25%. Similar performance was observed in the case of Pt/TiO2 dosing on the photocatalytic degradation of phenol.[5] Interaction between the OH radical and phenol becomes enhanced by TiO2, so a sufficient amount of catalyst increases the formation of electrons or hole pairs, thereby generating OH radicals to increase photodegradation.[19] Also, an increased dose concentration resulted in the blockage of light penetration on the surface (shielding effect), which reduced the reaction rate. The negative effect of the catalyst concentration on the degradation efficiency may be due to the aggregation/agglomeration of catalyst that lowers the total number of active sites on the surface.[20] Therefore, increasing photocatalyst dosage properly could enhance the photocatalytic degradation effect because of the greater amount of active substance available to treat wastewater. The optimal dosage of 2 g L¡1 catalyst gave a better performance, and the catalyst dose of 2 g L¡1 was used for further studies.
3 2 1
60
0 0
20
40
60
80
100
Time (min)
40 20 0 0
50
100
150
200
250
300
Time (min)
120 (c)
5 mg L-1 10 mg L-1
100
TOC reduction (%)
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Photocatalytic degradation of phenolic contaminants in wastewater
30 mg L-1
80 60 40 20
Effect of the initial phenol concentration on photodegradation
0 0
Results of photodegradation of phenol at different initial concentrations (5–30 mg L¡1) using TiO2/UV are shown in Figure 3. It can be seen that when the initial phenol concentration was 5 mg L¡1 and 10 mg L¡1, it showed 100% and 98% phenol removal efficiency within 100 min, along with an increase in photocatalytic oxidation, yet the efficiency was drastically decreased at high concentration of 30 mg L¡1 (45%) (Fig. 3a). It was observed that at the equilibrium adsorption, the active catalyst sites are loaded with phenolic species. Hence, competitive adsorption of OH¡ on the same sites is less likely, which results in a decrease in the amount of OH¡ and O2¡ on the catalyst surface results, or reduced degradation efficiency.[20] The Langmuir–Hinshelwood model is usually used to explain the kinetics of photocatalytic reactions of aqueous solution containing organics. It is mainly to study the degradation rate and organic concentration. The rate of decrease of phenol concentration in solution is assumed to follow a first-order reaction as: [21] dc D kc ¡ dt
(1)
where C is the concentration of phenol in solution at a given time, t, and k is the reaction rate constant. Further, the integration to the preceding equation yields an
50
100
150
200
250
300
Time (min)
Fig. 3. (a) Effect of the initial concentration (5 mg L¡1 to 30 mg L¡1) on phenol removal; (b) Linear plot of time versus ln (C/ C0); (c) TOC removal depending on the initial concentration using TiO2/UV system.
exponential concentration profile with time, C D C0 exp. ¡ kt/
(2)
where Co, is the initial phenol concentration in solution. ln
C D ¡ kt C0
(3)
A plot of ln (C/C0) versus reaction time t yields a straight line, and the slope is the rate constant. The experimental data in Figure 3b was found to be fitted to the preceding equation and the value of the reaction rate constant, k with regression coefficient for phenol at different concentrations, as shown in Table 1. At the initial concentration of phenol (5 mg L¡1), the removal rate increased, but as
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Table 1. Effect of initial concentration of phenol on photocatalytic degradation of phenol using TiO2/UV system. Co (mg L¡1)
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5 10 30
Degradation ratio after 100 min (%)
K (min )
Correlation coefficient (R2)
100 98 45
0.0594 0.0269 0.0065
0.9701 0.9352 0.92661
¡1
the concentration increased (30 mg L¡1) the removal rate was reduced. This could be because as the initial concentration of phenol increases, a large number of phenol molecules is adsorbed on the surface of TiO2, which reduces the OH radical formation, since very few active sites remained for the adsorption of hydroxyl ions and OH radicals. From Table 1 it can be seen that the rate constant and the R2 value decreased with increasing phenol concentration. This could be due to the intermediates generated during the photocatalytic process that affect the overall phenol degradation rate. The attack of these radicals on organic compounds decreased, thereby decreasing photodegradation efficiency. The total organic carbon remaining in the solution after photocatalytic degradation of phenol by TiO2/UV for different phenol initial concentrations (5, 10 and 30 mg L¡1) is illustrated in Figure 3c. The amount of TOC removal is found to be best at lower phenol concentrations. As the concentration increased, the decrease in TOC removal showedpoor performance, which may be due to poorly formed OH radicals because of saturation. TOC removal is greatly accelerated with 90%, 83% and 35% for 5, 10 and 30 mg L¡1, respectively, of phenol concentration after 300 min of reaction. Thus, combining TiO2/UV enhances photodegradation of organic pollutants.
Change in pH during photocatalytic degradation of phenol Effects of changes in solution pH with time on the photocatalytic degradation of phenol (10 mg L¡1) has been studied (data not shown). In this experiment, the pH of the distilled water was 6.87; and on addition of phenol solution the pH was 5.62. But when TiO2 (2 g L¡1) was added the pH dropped to 4.32 after 5 h. Early in the reaction, pH was higher than 6.3, which is the pzc (point of zero charge) of TiO2, so that concentration of OH¡ ion is decreased by adsorption of OH¡ ions on the surface of the catalyst. As a result then, pH decreases. Additional decrease in pH after some reaction is due to OH¡ ions that continuously decrease from the oxidation reaction on the catalyst surface; byproducts are formed by degradation of phenol. Specifically, phenol is an acid that provides hydrogen ions in
a numeric field. The acid-base reaction is a very rapid reaction, and it can be represented by Eq. 4: ½H C ½P ¡ D Ka ½HP
(4)
where [HC] is hydrogen ion (mol L¡1), [PC] is phenol (mol L¡1), [HP] is; not dissociated phenol (mol L¡1), and Ka is acidity constant. Concentration of hydrogen ions in water ranges from approximately 10¡4 to 10¡10 mol L¡1.[22] Concentration of hydrogen ions is mainly represented by pH. When pH of the solution is the same as the pKa value of the acid, 50% of the acid provides hydrogen ions in solution in the form of anions. When pH value is greater than the pKa value, organic acid provides all the hydrogen ions which exist in the form of anions. Thus phenol is weakly acidic and so seems to decrease the pH of the solution. Effect of pH The hydrogen ion concentration of the aqueous solution largely influences the behavior of toxic organic pollutants. The surface of TiO2 photocatalyst used in the photocatalytic oxidation reaction depends on the pH of the aqueous solution. Generally, in TiO2 it was observed that it shows a pzc, which is not charged with electrical polarity when the pH of the aqueous solution is 6.3. Therefore, when the pH of the solution is low (acidic), HC ions are adsorbed on the catalyst surface and have the properties of the anode. Conversely, at high pH (basic), OH¡ ions adsorbed on the surface become cathodic.[23,24] The pzc of Degussa P25 that was used in this study is also of pH 6.3. Thus it can be seen that where the pH > 6.3, the surface of the TiO2 interferes with the activity of OH¡ radicals, extruding the negative ions, which become significantly negative charge. By contrast, in the case of pH < 6.3, to attack the negative ions to hydrate the surface of TiO2, organic matter attains a significantly positive charge.[12] TiIV ¡ OH C H C ! TiIV ¡ OH2
(5)
TiIV ¡ OH C OH ¡ ! TiIV ¡ O ¡ C H2 O
(6)
Thus, the pH of the solution changes the surface of TiO2 catalyst, which is dispersed in aqueous solution and is characteristic of adsorption and desorption, which influences the speed of production of OH radical as a result of the overall reaction rate. Figure 4a shows the effect of different solution pHs with time on the photocatalytic degradation of phenol over TiO2 with UV light. The phenol removal was gradually increased, with time up to 100 min for a wide range of pH values. It was found that increase in solution pH increases the percentage of degradation The maximum percentage
Photocatalytic degradation of phenolic contaminants in wastewater 100
(a)
pH 3 pH 7 pH 9 pH 11
C/Co (%)
80
60
40
20
0 0
20
40
60
80
100
120
Degradation Efficiency (%)
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Time (min) (b) 10 min 50 min 100 min
100
60 40 20 0 4
The lowering of the degradation percentage with increasing pH can be described by the amphoteric nature of TiO2 in aqueous solution. Figure 4b shows the effects of phenol removal when TiO2 2 g L¡1 in 10 mg L¡1 phenol. From the results, pH 9 shows the best effects; however, pH 11 shows the opposite result, so we assume it is because of the OH group. Actually, the surface of the catalyst shows a negative change at high pH from the repulsive forces between phenol ions, which dissociate as anions in the solution and are difficult to adsorb on the active site of surface of TiO2, thereby decreasing the reaction rate. Also hydroxyl ion creates a strong oxidative OH radical by capturing the hole (hC) formed on the surface of TiO2. The proper number of protons are needed in the photoreduction reaction using TiO2. When there are too many protons, TiO2 becomes acid, and so electron (e¡) holes will recombine rapidly.
Effect of temperature
80
2
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6
8
10
12
pH
Fig. 4. Degradation of phenol (10 mg L¡1) (a) depending on the initial pH; (b) degradation efficiency of phenol according to the initial pH (3 to 11) using TiO2/UV system.
degradation values of phenol were found to be 68.21%, 78.14%, 98.00%, and 80.42% with solution pH values of 3, 7, 9, and 11, respectively. At lower pH, decrease in phenol degradation may be due to the decrease in hydroxyl ions that are required to react with valence band hole to form OH radicals.[5] However, the degradation of phenol in solutions was enhanced in the alkaline medium rather than in neutral or acid conditions, and the optimum pH value was 9 with maximum removal of phenol (98%). Bahemamm et al.[25] showed that when the intensity of light is not strong, the change in pH strongly affected the functional groups of the TiO2 surface in the acidic region by the following equations:
The effects of temperature on photocatalytic degradation of phenol were investigated (Fig. 5). Although sometimes as the solution temperature increases the percent degradation of phenol is increased, this type of increase could be explained by the reverse reaction that hydrogen is oxidized into water by photocatalytic reaction, and also due to increase in the reaction rate taking place between the phenol molecules and the OH radicals. Matthews[26] reported that temperature increase enhances the reaction rate and consequently induces the decomposition reaction of organic matter. The performance was higher at higher temperature due to collision frequencies of phenol molecules.[27] In this study, temperature was regulated at 25, 40 and 60 C to study the effects of temperature on the reaction rate. The result of this study are shown in Figure 5. For the performance of phenol (10 mg L¡1), it was observed that almost complete decomposition occurs as temperature increased (Fig. 5a), so the efficiency of the reaction cannot be understood. Further the reaction applied for phenol (50 mg L¡1), due to higher concentration, the reaction was hard to observe clearly; however, it can be seen that as time passes, the efficiency in phenol remove decreased at higher concentrations (Fig. 5b). However the optimum temperature in our study was observed to be 60 C. This is because as the temperature increases, the adsorption rate constant or surface reaction rate on the catalyst surface increases.
Effect of H2O2 TiOH C H C $ TiOH2C .acidic condition/
(7)
TiOH C OH ¡ $ TiO ¡ C H2 O .alkaline condition/ (8)
An advanced oxidation process using UV/H2O2 is another way to degrade phenolic compounds in aqueous solutions.[28] The UV/H2O2 effect results in formation of OH radicals, which accelerate the oxidative degradation of the phenolic
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100
(a)
80
120 o 25 C o 40 C o 60 C
TiO2/UV+0.3 mM H2O2 TiO2/UV+ 0.15 mM H2O2 TiO2/UV+ 0.1 mM H2O2 TiO2/UV+0.08 mM H2O2
100
UV+0.08 mM H2O2 UV
C/Co (%)
C/C0(%)
80 60
40
60 40 20
20
0 0
0 20
40
60
80
100
120
140
160
50
100
150
200
250
300
180
Time (min)
Time (min) 100
(b) 80
C/Co (%)
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0
o 25 C o 40 C o 60 C
60
Fig. 6. Degradation of phenol depending on the various H2O2 (0.08 to 0.3 mM) using TiO2/UV system. Reaction conditions: (pH D 9, TiO2 D 2 g L¡1, phenol: 10 mg L¡1).
in the case of the degradation of 2-chlorophenol.[29,30] Equations for photocatalytic degradation involving H2O2 in the H2O2/UV system are presented as follows: [31]
40
H2 O2 C hv ! 2OH
(9)
20
h C C OH ¡ ! OH ¡
(10)
e ¡ C O2 ! O2¡
0 0
100
200
300
400
500
¡
e C H2 O2 ! OH C OH
(11) ¡
(12)
Time (min)
Fig. 5. Degradation of phenol (a) depending on the temperature at 10 mg L¡1 phenol and (b) at 50 mg L¡1phenol using TiO2/ UV system.
compounds in water. The problem of high cost of H2O2 and the low oxidation rate of TiO2 can be solved by combining both systems. The effect of H2O2 addition on phenol photodecomposition in the presence of TiO2/UV was also investigated. Figure 6 compares the photodegradation of 10 mg L¡1 phenol in aqueous solutions under various reaction conditions. Photocatalytic degradation of phenol by TiO2/UV (TiO2; 2 g L¡1; 200 W) showed 98% of phenol removal. When the reaction was carried out in presence of TiO2/ UV/H2O2 the performance was better than the TiO2/UV. So, the experiments were carried out in various concentration of H2O2 addition to the reaction for TiO2/UV/H2O2. Thus for the system containing TiO2/UV/H2O2, enhancement in the phenol removal was observed with increase in the H2O2 concentration. The phenol removal improved and complete degradation was observed at higher concentration of H2O2 (0.3 mM). The removal efficiency increased up to 100% when H2O2 was added to the UV/ TiO2 system. A similar type of performance was also seen
Phenolic compound C OH ! H2 O C R
(13)
! further oxidation Photolysis by H2O2/UV generates OH radicals from Eq. 9 in presence of UV light promoting the rate of reaction. It was reported that H2O2 is a stronger electron acceptor than oxygen and thus prevents the recombination between holes/electrons, which means that the OH radical generated by H2O2 reacts with holes, decreasing the chances of holes/electron recombination, as shown in Eq. 12. However by producing electrons which is directly used in oxidation reaction of phenol, the rate of phenol photodegradation increased significantly in the TiO2/UV/H2O2 system. In the case of a real wastewater study using a TiO2/UV system and TiO2/UV/H2O2 (0.3 mM) system shows 96.7% and 98.4% after 200 min of reaction, respectively (Table 2). However, the performance of a photocatalytic system using TiO2/UV as a photocatalyst in presence/ absence of H2O2 with our system for phenol degradation has been systematically compared with ones reported earlier, indicating the photocatalyst tested in this study showed fairly good performance for significant phenol removal (Table 3).
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Table 2. Performance of TiO2/UV system in presence of different ions for the photocatalytic degradation of phenol containing synthetic wastewater (SW) and real wastewater (RW). Synthetic wastewater (SW) Parameters studied TiO2/UV TiO2/UV/H2O2 (0.3 mM) TiO2/UV/NaCl (1 mM) TiO2/UV/CHCl3 (1 mM) TiO2/UV/K2Cr2O7 (0.07 mM) TiO2/UV/K2Cr2O7 (0.2 mM)
Time (min)
Degradation (%)
Time (min)
Degradation (%)
100 100 300 200 300 300
98 § 0.74 100 § 0.82 100 § 0.72 100 § 0.83 80 § 0.64 66 § 0.48
200 200 300 300 300 300
96.7 § 0.76 98.4 § 0.70 98.1 § 0.75 97.6 § 0.82 78.2 § 0.70 64.8 § 0.54
Effect of chloride ion Downloaded by [University of Connecticut] at 10:36 08 October 2014
Real wastewater (RW)
During photocatalytic reactions, ferric ion acts as electron acceptors that prevent the recombination of holes with electrons efficiently, thereby enhancing photocatalytic oxidation of phenol. Chloride ion is also a substance that acts as an electron donor like HO2 and H2O2, but Cl¡ sometimes hinders and reduces catalytic activation by adsorption to the active sites. Detailed descriptions for chloride ions that scavenge the photonproduced holes and OH radicals more effectively have been reported.[32] The effects of different chloride ions on photocatalytic decomposition of phenol were studied in synthetic wastewater (SW) and real wastewater (RW) (Figs. 7a and b). Different chloride sources such and NaCl and CHCl3 (1 mM each) were used to study their effects, and the results showed very similar trends with that of the TiO2/ UV system. Addition of chloride ion shows no significant effect on the observed photocatalytic decomposition of phenol. It has been reported that at higher pH the negatively charged catalyst surface repelled the coming of chloride ions to the surface and restricted their adsorption without influencing the decomposition of organic compound.[33] The other reason could be that during reaction; chloride ions along with free radicals decreased the chances of the reactive reactant molecule and free radical and
thus lowered the performance in presence of chloride ions. Literature studies also observed negative effect of chloride ions for the degradation of 4-chlorophenol and phenol.[34,35] The foregoing reasons suggest that chloride ions in the aqueous solution could act as electron scavengers, competing with molecular oxygen for the active site, thereby lowering the reaction rate.
Effect of chromium ion In this study, as with the effects of chloride ions, of the presence of selected chromium Cr (VI) (Cr2O72¡) metal ion was also examined. It has been reported that metal ions such as transition metal ions and noble metal ions deposited on a photocatalyst can increase the photocatalytic efficiency according to the redox process.[32] Generally it is considered that the metal sites act as trapping sites and function as electron acceptor and improves the photocatalytic degradation activity by impeding the recombination of holes with electrons. Figure 8a and b shows the variation in the concentration of chromium (0.07 and 0.2 mM K2Cr2O7) according to the reaction time in the presence and absence of Cr for SW and RW using a TiO2/UV system. Similar to the results of chloride, almost no induction in the efficiency of photodegradation of phenol was observed by the addition of Cr ion. The reason may be due to the
Table 3. Comparison of phenol degradation study by using TiO2 photocatalyst with literature under various conditions. TiO2 g L¡1
UV
H2O2 300 mM 10 mM 1.77 mM — —
7.5
300 W 17 W/m2 400 W 400W 260 W/m2 22.5 W/m2
2.0
200 W
0.3 mM
0.1 1.0 1.0 1.0 0.5*
*Pt/TiO2. nd: not determined.
—
Type of phenolic waste
Concentration (mg L¡1)
pH /Temp ( C)/Time (min)
Removal efficiency (%)
Phenol Phenol Phenol Phenol Phenol and 2-cholrophenol 2-chloro and 2-nitrophenol Phenol
100 10 50 10 50
8 /nd/ 60 7 / 25 / 120 6.8 / 25 / 180 6.7–7.2 /32/ 300 3 /25 / 180
[36] [37] [29] [38] [5]
10
3 /25 / 140 min
99.2 97.0 94.0 99.0 87.0 100 95.0
10
9 /60 / 100
100
This study
Reference
[33]
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120 (a)
TiO2/UV + S.W. TiO2/UV +1 mM NaCl+ S.W.
100
0
C/C (%)
0
C/C (%)
TiO2/UV +S.W.+ 0.2 mM K2Cr2O7
80
60
60
40
40
20
20 0 120
0 120
(b)
TiO2/UV + R.W.
(b)
TiO2/UV + R.W.+ 0.07 mM K2Cr2O7
TiO2/UV + S.W. TiO2/UV + 1mM CHCl3+S.W.
TiO2/UV + R.W.+ 0.2 mM K2Cr2O7
100
TiO2/UV +R.W. TiO2/UV + 1mM CHCl3+R.W.
C/C (%)
100
0
80
0
C/C (%)
TiO2/UV + S.W. TiO2/UV + S.W.+ 0.07 mM K2Cr2O7
100
TiO2/UV + R.W. TiO2/UV+1 mM NaCl+R.W.
80
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(a)
60
80 60 40
40 20
20 0 0
0 0
50
100
150
200
250
Time (min)
Fig. 7. Effect of chlorides on TiO2/UV system for the removal of phenol in synthetic (SW) and real wastewater (RW) (a) NaCl (1 mM); (b) CHCl3 (1 mM). Reaction conditions: (pH D 9, TiO2 D 2 g L¡1, phenol: 10 mg L¡1).
addition of smaller amounts of Cr in aqueous solution, and another may be the difficulty of reduction of Cr (VI) to Cr (III) because of its highly reactive oxidation/reduction potentials under given conditions. Thus the photocatalytic activity was not significantly affected by the addition of chloride and metal ions that may have inhibited the formation of OH¡, preventing phenolic molecules from reaching the surface active site. The TiO2 surface is negatively charged and repulsive forces will lead to decreased adsorption at high pH.
Removal of phenol from real wastewater by TiO2/UV system Experiments were carried out to find the effectiveness of the TiO2/UV system for removing phenol from real wastewater. Experiments were conducted partly to examine the possible differences in degradation features for real wastewater compared to the results from a synthetically prepared phenol aqueous solution. It was observed that when the TiO2/ UV system was used for photocatalytic degradation, the
50
100
150
200
250
300
Time (min)
300
Fig. 8. Effect of chromium (0.07 mM K2Cr2O7 and 0.2 mM K2Cr2O7) ions on TiO2/UV system for the removal of phenol in (a) synthetic wastewater (SW) and (b) real wastewater (RW). Reaction conditions: (pH D 9, Phenol: 10 mg L¡1; TiO2: 2 g L¡1).
treatment efficiency for real wastewater was found to be lower (96.7%) compared to artificial wastewater (98%) after 200 min. The real wastewater employed in the experiments was thought to contain some other organic substances, which may have led to the decrease in the degradation of phenol efficiency. This was also probably due to the interfering effects of the coexisting substances on the degradation of phenol. The addition of H2O2, chloride and chromium ions to increase the treatment efficiency was also studied. Treatment efficiency excluding H2O2 showed no significant effect on the treatment efficiency in the presence of chloride ions and metal ions (Table 2). The removal and or treatment efficiency of phenol synthetic wastewater was found to be very comparable with that obtained phenol from wastewater samples (Table 2).
Conclusion Efficiency of photocatalytic degradation of phenol by using a TiO2/UV system in aqueous solution was studied.
Photocatalytic degradation of phenolic contaminants in wastewater
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The TiO2/UV system showed the highest efficiency towards phenol degradation compared to catalyst alone or direct photolysis (UV alone), at similar experimental conditions. Significant removal of phenol (10 mg L¡1) was observed at photocatalyst (2 g L¡1), at alkaline pH 9.0 and temperature at 60 C within 100 min with higher mineralization in terms of TOC. The addition of chloride ions and metal ions showed no significant response for phenol degradation. Addition of H2O2 improved the degradation efficiency, and complete removal of phenol was observed. The TiO2/UV system used to evaluated for phenol degradation of systems containing real wastewater exhibited significant degradation performance and was determined to have strong practical applications.
[11]
[12] [13]
[14]
[15]
[16]
Funding [17]
The project was supported by the Fundamental Technology R & D Program for Society of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant number: 2013 M3C8A3078596 and 2009-0083527).
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